Plants must supply their various tissues with the carbohydrates they produce through photosynthesis in the leaves. However, they do not have a muscular pump like the human heart to help transport this vital fuel. Instead, they use pump proteins in their cell membranes for this purpose. Together with colleagues from the Carnegie Institution for Science in California, Alisdair Fernie from the Max Planck Institute of Molecular Plant Physiology in Potsdam has identified a hitherto unknown protein in the carbohydrate transport chain. The researchers' discovery could help to protect plants against pests and increase harvest yields.
Vascular pathways consisting of interconnected cells act as a system for the transport of carbohydrates in plants. Phloem, the tissue that carries the nutrients, consists of, among other things, the actual conducting cells, which are also known as sieve elements, as well as the surrounding companion and phloem parenchyma cells. Carbohydrates are mainly transported in the phloem in the form of sucrose. The cell membrane of the sieve cells contains pump proteins that actively convey sucrose into the vascular pathways. Up to now it was unclear how the sucrose travelled from the parenchyma cells to the transport pumps, the sieve elements. Thus, information about an important element in the transport chain was missing.
With the participation of the Max Planck Institute of Molecular Physiology, a research group at the Carnegie Institution for Science in Stanford, USA, has now succeeded in identifying this previously unknown sucrose transporter. Different proteins are involved here which belong to the recently identified protein family known as SWEET. The SWEETs arise in the cell membrane of the phloem parenchyma cells. They act as molecular pumps that convey the sucrose out of the parenchyma cells and forward it to a second transport system -- identified by the group 20 years ago -- which feeds the sucrose into the actual phloem conducting cells.
In their studies, the researchers examined sucrose transport in the thale cress, Arabidopsis thaliana, and in rice plants. In order to track down the function of the SWEET proteins, they switched off the corresponding genes in a series of plants. This enabled them to discover that, when their SWEETs do not work, plants have a considerably higher sucrose content in their leaves. "Because the sugar cannot be transported away, it accumulates in the leaf tissue, and other parts of the plant, like the roots and seeds, do not receive an adequate supply of sucrose," explains Alisdair Fernie from the Potsdam-based Max Planck Institute.
This discovery represents an important development for plant breeding as, in many cases, the parts of plants used by humans, like seeds and tubers, do not form any carbohydrates themselves but are, instead, supplied by the leaves. "We can now regulate these molecular pumps precisely and thereby increase the transport of sucrose to the plant seeds. One day, it may be possible to increase the harvest yield of agricultural crops in this way," explains Wolf Frommer from the Carnegie Institution. Moreover, the SWEETs provide a promising starting point for the protection of plants against pest infestations. Some pests, for example the bacterium Xanthomonas oryzae, which causes leaf streak in rice, misuse these transporters to access the plant's sucrose and feed on it themselves. For this reason, the scientists would now like to clarify the role of these transporters in pest infestation in greater detail.
The researchers suspect, moreover, that the corresponding pump proteins have a similar function in humans and animals. If confirmed, this would constitute a very important discovery for diabetes and obesity research, as the identity of the protein responsible for the transport of carbohydrate from the intestine into the blood and from liver cells is not yet known.
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